The present invention relates a polyethylene microporous film for a rechargeable battery separator and a method of preparing the same, more particularly, the present invention relates the polyethylene microporous film, which has a laminated structure comprising B layer/A layer/B layer, formed by melt-mixing with a polyethylene and an aliphatic hydrocarbon solvent together at controlled mixing ratios to separately form an A layer and a B layer having different porosities, and then coextruding the A and B layers, thus exhibiting excellent mechanical properties, such as strength and elongation, and high-temperature stability, and the method of preparing such a polyethylene microporous film.
As various portable apparatuses, including mobile phones, notebook PCs, portable videos, PDAs, or portable multimedia players, have trended toward small sizes and light weights, rechargeable battery markets have gradually increased. A rechargeable battery is a chemical cell, which can be semi-permanently used through continuous repetitive charges and discharges using an electrochemical reaction, and has been realized as a lead storage battery, a nickel cadmium battery, a nickel hydrogen metal battery, a lithium ion battery, a lithium ion polymer battery, etc. In particular, of these batteries, a lithium ion battery and a lithium ion polymer battery, each of which has high voltage and excellent energy density properties, lead the rechargeable battery markets.
A lithium rechargeable battery comprises an anode, a cathode and a separator interposed between the anode and the cathode so as not to physically contact them. The separator for a lithium ion battery must have mechanical strength to endure a high-speed winding process upon manufacture of a battery, be chemically stable in an electrolyte, and have high-temperature stability to prevent the generation of short circuits and overcharge. As well, high capacity, excellent battery properties, stability and high productivity have been required in recent years.
Presently, as a separator for a lithium ion battery, a polyethylene microporous film is typically used. When the polyethylene microporous film is biaxially stretched, it has superior mechanical strength and remains chemically stable. In this way, efforts to improve properties required for the use of the polyethylene microporous film as a separator for a lithium ion battery have been thoroughly conducted.
In this regard, Japanese Patent No. 1848017 discloses a method of preparing a polyethylene microporous film, which comprises dissolving polyethylene having average weight molecular weight (Mw) ranging from 500,000 to 1,500,000 in a solvent using heat to prepare a solution, forming the solution into a gel sheet, removing the solvent from the gel sheet so that the solvent is present in the gel sheet in a proportion of 10-80 wt %, heat-stretching the gel sheet, and then removing residual solvent. Japanese Patent No. 2126761 discloses a polyethylene microporous film, prepared using polyethylene having an Mw of 500,000 or more.
In addition, Japanese Patent No. 1759736 or 1918760 discloses an ultrahigh molecular weight α-olefin polymer microporous film and a method of preparing the same, the method comprising preparing a solution of α-olefin polymer having an Mw of 500,000 or more into a gel body, removing at least 10 wt % of solvent from the gel body so that the amount of the α-olefin polymer in the gel body is 10˜90 wt %, stretching the gel body at a temperature further increased to 10° C. higher than the melting point of the a-olefin polymer or lower, and then removing residual solvent. In addition, Japanese Patent No. 1948121 discloses a polyethylene microporous film, prepared by forming a solution of polyethylene having an Mw of 500,000 or more into a gel body, removing the solvent from the gel body so that the solvent is present in the gel body in a proportion of 80˜95 wt %, uniaxially stretching the gel body at least twofold thus causing the magnification area thereof to be increased by at least ten times at 120° C. or less, and then removing residual solvent. In addition, Japanese Patent No. 1866164 discloses a method of preparing a polyolefin microporous film, comprising loading a solution containing ultrahigh molecular weight polyolefin having an Mw of 500,000 or more into a die, extruding the solution from the die while quenching it to 90° C. or less at a cooling rate of 50° C./min to form a gel body, removing at least 10 wt % of solvent from the gel body so that the amount of ultrahigh molecular weight polyolefin in the gel body is 10˜90 wt %, stretching the gel body at a temperature further increased to 10° C. higher than the melting point of ultrahigh molecular weight polyolefin or lower, and then removing residual solvent.
Further, Japanese Patent No. 2132327 discloses a polyolefin microporous film having a porosity of 35˜95%, an average pore diameter of 0.001˜0.2 μm and a fracture strength of 0.2 kg or more at a width of 15 mm, prepared using a polyolefin composition including 1 wt % or more of ultrahigh molecular weight polyolefin having an Mw of 700,000 or more, and a ratio of average weight molecular weight/number average molecular weight (Mw/Mn) of 10˜300.
Further, Japanese Patent No. 2657434 discloses a polyethylene microporous film and a method of preparing the same, the method comprising mixing 10˜50 wt % of a composition, including 1˜69 wt % of ultrahigh molecular weight polyethylene having an Mw of 700,000 or more, 98˜1 wt % of high density polyethylene, and 1˜30 wt % of low density polyethylene, in which an Mw/Mn ratio of the component containing ultrahigh molecular weight polyethylene and high density polyethylene is 10˜300, with 50˜90 wt % of a solvent to prepare a solution, extruding the solution from a die on a cooling roll to form a gel composition, and then stretching the gel composition at a temperature further increased to 10° C. higher than the melting point of the polyethylene composition or lower. As such, the polyethylene microporous film has a thickness of 0.1˜50 μm, a porosity of 35˜95%, an average pore diameter of 0.001˜1 μm, tensile fracture strength of 200 kg/cm2 or more, and impermeability at temperatures less than 135° C.
Also, Japanese Patent No. 3351940 discloses a polyolefin microporous film for a separator of a lithium ion battery, the film being prepared using a solution comprising 5˜50 wt % of polyolefin, having an Mw ranging from 500,000 to 2,500,000 and an Mw/Mn ratio less than 10, and 95˜50 wt % of a solvent.
In addition, Japanese Patent No. 2794179 discloses a polyethylene microporous film, prepared using high density polyethylene having an Mw ranging from 400,000 to 2,000,000 and an Mw/Mn ratio of 25 or less, and Japanese Patent No. 2961387 discloses a polyethylene microporous film, prepared using polyethylene having a viscosity average molecular weight ranging from 160,000 to 2,000,000, and Japanese Patent No. 3121047 discloses a microporous film having a three-dimensional network structure, comprising at least 30 wt % of ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 2,000,000 or more based on the amount of microporous film. Additionally, various techniques are reported to control the properties of a polyethylene microporous film by adjusting the molecular weight and the amount of ultrahigh molecular weight polyethylene having an Mw of 1,000,000 or more [Japanese Patent Nos. 3258737, 3333287, 3497569, and 3486785], in which the polyethylene microporous film has a monolayer structure.
On the other hand, Japanese Patent No. 3195120 discloses a microporous film having a vein-shaped open pore structure comprising microfibrils, having a porosity of 30˜70%, tensile strength of 1,000 kg/cm2 or more and an average micropore diameter of 0.1˜3 μm, prepared using high molecular weight polyethylene having a limiting viscosity [η] of 5 dl/g or more.
Korean Patent No. 371390 discloses a separator having a multi-layered structure, formed by laminating a plurality of polymer layers including a) a polypropylene layer, b) a polyethylene layer, and c) a tie layer, including polypropylene having an electrophilic functional group and polyethylene having a nucleophilic functional group, which is chemically bonded therewith.
Korean Patent No. 409019 discloses a multi-layered microporous film, comprising a) a support layer of polymer film, having a pore size of 0.001˜100 μm and a thickness of 1˜50 μm, and b) a shutdown layer formed on either one or both surfaces of the support layer, including a polymer having a melting point 40˜75° C. lower than that of the polymer of the support layer, and having a pore size of 0.001˜100 μm and a thickness of 0.01˜20 μm, and a method of preparing such a microporous film.
In addition, Korean Patent No. 263919 discloses a microporous laminated film, which comprises a first polymer layer, including a polyethylene copolymer, and a second polymer layer, including a polypropylene copolymer and formed on at least one surface of the first polymer layer, and a method of preparing such a film. As such, the polyethylene copolymer results from copolymerization of polyethylene, including 10 wt % or less of polyethylene having a molecular weight of 1,000,000 or more and 60 wt % or less of polyethylene having a molecular weight of 10,000 or less, and any one or more selected from the group consisting of methylpentene, propylene, butene, pentene, hexene, and octene. The polypropylene copolymer is obtained by copolymerzing polypropylene, including 30 wt % or less of polypropylene having a molecular weight of 1,000,000 or more and 40 wt % or less of polypropylene having a molecular weight of 10,000 or less, and any one or more selected from the group consisting of methylpentene, propylene, butene, pentene, hexene, and octane.
In addition, Korean Patent Laid-open Publication No. 2000-51312 discloses a polypropylene microporous film for a battery separator, comprising 25˜55 wt % of a polypropylene homopolymer having a melt flow index of 0.1˜2.0 g/10 min, 3˜30 wt % of polypropylene terpolymer having a melt flow index of 3˜10 g/10 min, 30˜70 wt % of paraffin oil, and 0.05˜0.2 wt % of an antioxidant. Also, Korean Patent Laid-open Publication No. 2000-51313 discloses a polyethylene microporous film for a battery separator, comprising 20˜40 wt % of high density polyethylene having a melt flow index of 0.2˜0.5 g/10 min and a density of 0.960˜0.969 g/cm3, 4˜20 wt % of high density polyethylene having a melt flow index of 0.02˜0.1 g/10 min and a density of 0.950˜0.958 g/cm3, 40˜70 wt % of paraffin oil, 5˜15 wt % of DOP plasticizer, 0.1˜0.5 wt % of a nucleation agent, and 0.1˜0.5 wt % of an antioxidant. However, a plurality of separators for lithium ion rechargeable batteries, which is conventionally used, has not yet overcome difficulties in being manufactured into a thin film due to decreasing mechanical strength when permeability is increased, or due to decreasing permeability when mechanical strength is increased, from the point of view of the physical properties of the microporous film.
As mentioned above, almost all of the microporous films are mainly composed of a polyethylene. Hence, when current is drastically increased due to external or internal short circuits, the internal temperature of the battery drastically increases. Eventually, the microporous film of the battery may become deformed, and thus the stability of the battery is difficult to maintain. To overcome such problems, methods of preparing a microporous film comprising a polyethylene layer and a polypropylene layer laminated thereon or of variously coating the surface of the microporous film have been reported. However, these methods are disadvantageous because additional processes are required, causing other problems. That is, when the polypropylene layer is laminated by a dry process, film properties may be degraded or become non-uniform. Likewise, the coating process may cause problems with the stability of a battery, on account of the possibility of contamination. Therefore, a polyethylene film for a separator having higher stability is urgently required.